Polyolefins have acquired a global status for itself in every field. The demand for polymers based on olefins has increased whether it is commodity or niche products. There is always a craving for better product hence a requirement of continuous improvement in Ziegler-Natta catalyst system as well as polymerization processes is there. Production of polyolefins having desired properties like narrow MWD for non woven or broad MWD for pipe, blow molding and films, high isotacticity, better comonomer incorporation are always in demand.
Ziegler-Natta catalyst system typically consists of transition metal halide normally titanium halide supported on metal compound which is typically magnesium dihalide. Along with transition metal, also there is organic component known as internal electron donor, which has typical role to play during catalyst synthesis and polymerization. The organic electron donors (Lewis base) that are being used as internal donors are organic compounds having oxygen, nitrogen, phosphorous and/or silicon components which may be in the form of acids, alcohols, esters, anhydrides, ketones etc. The current state of the art internal donors are organic compounds such as esters of phthalates, alkyl benzoate, succinates and 1,3-diethers which impart certain characteristics due to their chemical and electronic environment that enhances catalyst activity while imparting improved product properties like high melt flow index, good bulk density, narrow/broad molecular weight distributions and high isotacticity.
The role of internal donors in Ziegler-Natta catalyst is to stabilize the primary crystallites of magnesium dihalide which is generated in-situ. Apart from this, the internal donor also being better Lewis base have preferred coordination with the higher acidity coordination sites on magnesium dihalide matrix which in turn avoid the coordination of titanium and hence prevents the formation of inactive sites. They also increase the activity of low active sites.
WO/2012/118883 describes the processes for preparing procatalyst compositions where multiple contact steps with a halogenating agent are involved in the presence of a substituted phenylene aromatic diester and at least one other internal electron donor. The halogenating agent is a titanium halide having the formula Ti(ORe)fXh wherein Re and X are defined as above, f is an integer from 0 to 3; h is an integer from 1 to 4; and f+h is 4. The multi-contact procatalyst compositions produced from the processes improve polymer properties specifically it improves polymer bulk density. The procatalyst precursor described in this invention may be a magnesium moiety compound (MagMo), a mixed magnesium titanium compound (MagTi), or a benzoate-containing magnesium chloride compound (BenMag).
WO/2010/078494 describes procatalyst compositions having an internal electron donor based on substituted phenylene aromatic diester and optionally an electron donor component. Ziegler-Natta catalyst compositions containing the present procatalyst compositions exhibit high activity and produce propylene-based olefins with broad molecular weight distribution.
US20110152481 relates to catalyst systems containing solid catalyst components comprising titanium, magnesium, halogen and 1,8-naphthyl diaryloate internal electron donor compound; organoaluminum compounds and alkyl benzoate derivatives as external electron donors. The present invention also provides methods of making the catalyst systems, and methods of polymerizing or copolymerizing alpha-olefins using the catalyst systems.
Ziegler-Natta catalyst system has gone through continuous improvement in terms of better catalyst activity in olefin polymerization and at the same time production of polymer products having desirable properties with more simple and cleaner manner.